New frontiers in radiobiology: in vitro and in vivo exploration of the flash effect.

Ocular melanoma is a rare but aggressive malignancy, for which the primary treatment modality remains radiotherapy. However, conventional radiotherapy is associated with significant side effects, primarily due to collateral damage to surrounding ocular structures, including the retinal pigment epithelium (RPE). Therefore, there is an urgent need for novel irradiation approaches that can minimize these adverse effects while maintaining therapeutic efficacy. In this study, we explore the potential of electron FLASH as an alternative to conventional irradiation, hypothesizing that it could reduce side effects while preserving its antitumor efficacy. As an initial step, we investigate the effects of electron FLASH irradiation on ARPE-19 cells, a human RPE cell line, based on the rationale the RPE is inevitably exposed during ocular melanoma radiotherapy. Additionally, we aim to mitigate radiotherapy-induced oxidative stress and inflammation by evaluating two novel dual-action biosteroids. These compounds retain the well- established anti-inflammatory properties of corticosteroids while, through biomodification, acquiring antioxidant capabilities. Their protective effects were assessed on ARPE-19 cells to determine their potential as therapeutic adjuvants. Building upon these in vitro investigations, we further tested our hypothesis in an in vivo wild-type (C57Bl/6) mouse model. Various parameters were analyzed to evaluate the biological response to electron FLASH in comparison to conventional radiotherapy irradiation. Ultimately, we implemented an ocular melanoma mouse model to test the hypothesis of isoefficacy in tumor control between FLASH and CONV irradiation, favoring the optimization of new treatment strategies for this malignacy.